Mobile shipping container handlers are used worldwide and are manufactured by numerous companies. They are commonly found at rail intermodal yards or container shipping ports. Operation of a container handler is performed by a person referred to as the operator. The operator uses various controls including a joystick to maneuver the spreader. Over the years, safeties have been built into the machine using electric relays and timers and/or computer systems, such as a PLC (Programmable Logic Control), to decrease the number of accidents caused by operator error. The safety logic surrounding hoist function is typically limited based on the following situations:
1) Is the machine locked onto a container? (Green light)
2) Is the machine unlocked from a container? (Red light)
3) Is the machine within its safety envelope?
The locked and unlocked information is determined by proximity sensors on the twistlocks that are located in the four corners of the spreader (some older machines use only two sensors, one for each arm). The machine safety envelope is determined based on the type of machine. On a typical top pick, the safety envelope is simply the maximum capacity of the machine and weight is measured with a load cell or a pressure sensor on one or both of the lift cylinders. On a reach stacker, the safety envelope is more complex due to significant changes in the center of gravity caused by the reach capability of the machine. The envelope is determined based on three sensor readings. These readings are:
a) Boom angle—Inclinometer sensor
b) Boom length—Cable reel with a potentiometer
c) Weight of the load—Load cells or pressure sensors on the lift cylinders
A calculation of the boom angle and length readings is performed repeatedly and the maximum lift capacity is adjusted based on that calculation. The weight of the load is then logically compared to the maximum lift capacity to ensure that the machine is still within its safety envelope. If any one of the aforementioned situations are not met, boom/mast function is limited. In the event of situation 1 or 2 failing to be met, all boom/mast hoist and lower functions are locked. In the event of situation 3 failure, boom/mast hoist is locked.